362 PAST succession: the ceneosere. 



Carylus, Crataegus, Kalmia, Myrica, Prunus, Quercus, Rhamnus, Rhus, and 

 Vxhumum. 



As to the dominant trees, and especially the climax ones, the wide range of 

 genera which are now boreal, temperate, or tropical leads irresistibly to the 

 conclusion that climaxes were already differentiated in some degree. The 

 presence of a great mediterranean sea in and about a mountain region suggests 

 an altitudinal differentiation of climate with something of the sharp contrasts 

 noted on peaks in the tropics. Moreover, while the shores and lowlands of 

 the Cretaceous Mediterranean possessed an oceanic climate which permitted 

 the migration of tropical and subtropical genera far to the north, it seems 

 quite possible that the moimtain and basin regions had developed a cooler and 

 drier climate, which approached the continental type. The alternative 

 assimiption that the dominant genera of trees were widely and uniformly dis- 

 tributed, and that there was no essential variation in a stretch of 35° of latitude in 

 both North America and Europe (Chamberlin and Salisbury, 1906 : 3 : 175) even 

 is attractive because of its simplicity. However, the difficulties of its unques- 

 tioned acceptance are steadily increasing. The growing conviction of exten- 

 sive climatic differentiation since the Permian, if not since the Cambrian, 

 furnishes perhaps the chief objection. Having been routed from the Paleo- 

 phjrtic, the earlier ideas of a uniform climate have found a refuge in the Ceno- 

 phytic. Even this refuge is a vain one, however, if the doctrine of the imity 

 of processes is to be applied throughout, as the newer investigations demand. 

 Upon this theoretical basis it seems quite impossible to interpolate a Cretaceous- 

 Eocene uniformity of climate and vegetation between the sharp differentiations 

 of the Permian-Triassic and the Pliocene-Pleistocene. There is no question 

 of the existence of great readjustments after these crises of differentiation, 

 but the assumption that they led to widespread and complete xmif ormity does 

 not seem admissible. The process of differentiation in vegetation is also 

 against this assumption. A uniform mixture of dominants implies a similar 

 response to climate, and hence similarity of behavior in the face of changing 

 climate. It seems impossible that genera which we now know as boreal, 

 temperate, and tropical should have existed in the most complex and uniform 

 mixture through a vast region characterized by a warm climate, and then 

 have been completely differentiated by later climatic changes, along, but not 

 across, generic lines, into three great forest climaxes. The phylogenetic rela- 

 tions of the boreal coniferous forests of to-day are strongly against this assump- 

 tion. The presence of related species of the same genera, such as Picea, Abies, 

 Pinus, and Larix, in Eurasia and North America, in the northeast, the Rocky 

 Mountains, and the northwest, shows that the action of a climatic change upon 

 a great vegetation mass is across generic lines, with the result that all the 

 major genera are represented in the new climaxes. Hence the assumption is 

 here advanced that this is the regular if not the universal course of differen- 

 tiation, and that in consequence Cretaceous vegetation must have exhibited 

 some segregation of genera into potential and probably actual climaxes. 



This conclusion is reinforced by the fundamental course of succession in 

 present-day vegetation. Even where a few dominant genera appear in the 

 climax stages, the effect of competition for water and light is to increase the 

 dominance of some at the expense of others, with the consequence that it is 

 rarely or never true that more than a few genera exist side by side as equivalent 



